Recently, the substitution of glass glazing with transparent materials which do not shatter or are more resistant to shattering than glass, has become widespread. For example, transparent glazing made from synthetic organic polymers is now utilized in public transportation vehicles, such as trains, buses, taxis and airplanes. Lenses, such as for eyeglasses and other optical instruments, as well as glazing for large buildings, also employ shatter-resistant transparent plastics. The lighter weight of these plastics in comparison to glass is a further advantage, especially in the transportation industry where the weight of the vehicle is a major factor in its fuel economy.
While transparent plastics provide the major advantage of being more resistant to shattering than glass, a serious drawback lies in the ease with which these plastics mar and scratch, due to everyday contact with abrasives, such as dust, cleaning equipment and ordinary weathering. Continuous scratching and marring results in impaired visibility and poor aesthetics, and oftentimes requires replacement of the glazing or lens or the like.
One of the most promising and widely used transparent plastics for glazing is polycarbonate, such as that known as Lexan.RTM., sold by General Electric Company. It is a tough material, having high impact strength, high heat deflection temperature, good dimensional stability, as well as being self-extinguishing, and is easily fabricated. Acrylics, such as polymethymethacrylates, like Plexiglas, are also commonly and widely used glazing.
Attempts have been made to improve the abrasion resistance of these transparent plastics. For example, scratch resistant coatings formed from mixtures of silica, such as colloidal silica or silica gel, and hydrolyzable silanes in a hydrolysis medium, such as alcohol and water, are known. U.S. Pat. Nos. 3,708,225, 3,986,997, 3,976,497 and 4,159,206, for example, describe such compositions.
Copending commonly assigned U.S. application Ser. No. 964,910, filed Nov. 30, 1978, discloses another abrasion-resistant coating composition. This coating composition which is neutral to slightly basic, has been found to be highly desirable as a protective finish for plastic, as well as metal or metallized substrates. A particularly significant area of application for this coating is in the glazing and optical lens industry. Since these areas require a coating of high optical clarity, coatings which show flowmarks, dirtmarks or other marks which may impair visibility, are undesirable.
In copending commonly assigned U.S. application Ser. No. 964,911, filed on Nov. 30, 1978, and now U.S. Pat. No. 4,277,287, it is disclosed that the addition of a small amount of a polysiloxane polyether copolymer to the coating compositions disclosed therein eliminates the occurrence of undesirable flowmarks and the like, as well as providing other improvements in the hard abrasion resistant coating.
In many instances, however, in order to obtain good adhesion of the protective silicone resin coating composition to the substrate, a primer must first be applied to the substrate. This has been found to be especially necessary with cast acrylics, e.g., polymethylmethacrylates, like Plexiglas.
In copending U.S. application Ser. No. 34,164, filed Apr. 27, 1979, it is disclosed that if a solvent comprising a mixture of polar organic solvents miscible with water and alcohol and aggressive to plastic is used in the coating compositions of said U.S. Ser. No. 964,911, U.S. Pat. No. 4,277,287 instead of the dilution solvents, e.g., isobutanol, suggested therein, a protective coating composition is provided which adheres to cast acrylics without the necessity of a primer. The contents of the foregoing patents and applications are incorporated hereby by reference.
Now, surprisingly, it has been found that the need for special solvents and formula moidification can be eliminated, by employing a step comprising a wash coat of glacial acetic acid shortly prior to applying the final hard coat composition. As will be seen hereafter, the glacial acetic acid can be applied by common techniques such as flowcoating and dipcoating. The acid concentration and drainage time are important to the adhesion. Using acetic acid diluted with isopropanol reduces the ultimate abrasion resistance and acclerates delamination under RS Sunlamp exposure. The drainage time should be 2 to 5 minutes, since longer intervals between washing and coating cause poorer adhesion. Thus, primerless adhesion of the conventional silicone hard coat compositions to cast poly(methyl methacrylate) sheet is obtained according to this invention using simple procedures and without harm to the desired final properties, high abrasion resistance and excellent optical clarity.